System and method for removing narrowband noise

ABSTRACT

A system and method for removing narrowband noise from an input signal in which notch filters having notch frequencies corresponding to the noise are dynamically adjusted in accordance with a detected noise spectrum. The method may be applied to telemetry systems for implantable medical devices such as cardiac pacemakers to result in improved noise immunity.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a continuation of U.S. patent application Ser. No. 09/754,098, filed on Jan. 4, 2001, the specification of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention pertains to methods and systems for removing noise from signals. The invention finds particular application to telemetry systems used in implantable medical devices such as cardiac pacemakers and implantable cardioverter/defibrillators.

BACKGROUND

[0003] Implantable medical devices, including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, typically have the capability to communicate data with a device called an external programmer via a radio-frequency telemetry link. One use of such an external programmer is to program the operating parameters of an implanted medical device. For example, the pacing mode and other operating characteristics of a pacemaker are typically modified after implantation in this manner. Modern implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data which may typically be telemetered from an implantable device are various operating parameters and physiological data, the latter either collected in real-time or stored from previous monitoring operations.

[0004] Noise refers to any unwanted signal that interferes with the transmission and processing of data signals in a communications system. Such noise may arise from sources either internal or external to the system. Because of limited energy storage capability, implantable medical devices must necessarily transmit their data with a low signal energy, making the transmissions very susceptible to interference from noise. This means that an external programmer can only be satisfactorily used to receive data in relatively noise-free environments. Because of the widespread nature of electromagnetic noise sources, such a constraint may not only be inconvenient to the patient and clinician, but could also be hazardous in an emergency situation. Both broadband and narrowband noise sources contribute to the problem, with modern CRT monitors being a particularly common source of narrowband noise.

[0005] SUMMARY OF THE INVENTION

[0006] The present invention relates to a system and method for removing narrowband noise from a received signal. In a particular embodiment, after digitizing the received signal, narrowband noise is removed from the input signal samples with a series of notch filters having center notch frequencies generated adaptively so that the notch frequencies match the frequency peaks of a detected noise spectrum. The noise spectrum is detected by first computing a power spectrum of the input signal and then subtracting from it a template spectrum corresponding to an expected input signal without noise. A template spectrum is computed from a representative input signal generated by receiving a transmitted signal under noise-free conditions so that when it is subtracted from the input signal spectrum, the result approximates the power spectrum of the narrowband noise alone. In order to produce a detected noise spectrum that most closely approximates the true noise spectrum, the template spectrum is scaled by a factor that reduces the total power in the detected noise spectrum to a minimal value. The frequency peaks in the detected noise spectrum are then identified and used to synthesize filters with corresponding notch frequencies to remove the noise from the input signal.

[0007] The narrowband noise removal method may be employed in a system and method for receiving telemetry data from an implantable medical device to result in an improved capability for operating in noisy environments. In an exemplary system, the transmitted signal from the implantable device is a radiofrequency carrier waveform modulated with digitally encoded data in the form of transmit pulses. Further noise immunity may be provided to the system by matched filtering of the input signal samples and adaptive pulse detection.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a diagram of a telemetry system for an external programmer.

[0009]FIG. 2 is a diagram of the receiver portion of the telemetry system.

[0010]FIG. 3 is a diagram of the signal processing functions performed by the receiver.

DETAILED DESCRIPTION

[0011] Narrowband noise, such as that generated by certain electronic devices, is bandwidth-limited noise having a power spectrum with characteristic frequency peaks. Thus, a series of notch or bandstop filters with notch frequencies that correspond to those characteristic frequency peaks will remove the narrowband noise from an input signal in real time. Because the power spectrum of narrowband noise found in the environment is not constant, however, successful removal of such noise requires that the notch frequencies adapt to a changing noise spectrum. In accordance with the invention, a power spectrum corresponding to noise present within an input signal is detected by subtracting a template spectrum from the power spectrum of the input signal. The detected noise spectrum is then used to synthesize the notch filters that remove the noise from the input signal. By continuously or periodically detecting a noise spectrum from the input signal, the notch filters can be resynthesized with updated notch frequencies in near real-time to adaptively remove noise from the input signal in response to a changing noise spectrum.

[0012] The present invention can be applied to a telemetry data receiving system for an external programmer to result in improved performance in the presence of noise. Telemetry systems for implantable medical devices utilize radio-frequency energy to enable bidirectional communication between the implantable device and an external programmer. An exemplary telemetry system for an external programmer and a cardiac pacemaker is described in U.S. Pat. No. 4,562,841, issued to Brockway et al. and assigned to Cardiac Pacemakers, Inc., the disclosure of which is hereby incorporated by reference. A radio-frequency carrier is modulated with digital information, typically by amplitude shift keying where the presence or absence of pulses in the signal constitute binary symbols or bits. The external programmer transmits and receives the radio signal with an antenna incorporated into a wand which can be positioned in proximity to the implanted device. The implantable device transmits and receives the radio signal by means of an antenna, such as may be formed by a wire coil wrapped around the periphery of the inside of the device casing. As aforesaid, the limited energy storage capability of a typical cardiac rhythm management device necessitates that the signals transmitted from the implantable device be of low energy, thus decreasing the signal-to-noise ratio of the signal received by the external programmer.

[0013] In a particular implementation of a telemetry system, data generated by the implantable device is transmitted in the form of a carrier signal modulated with transmit pulses representing the encoded data. The received signal is digitized into input signal samples, and noise is removed from the samples by two filtering operations implemented in the digital signal processor, one for narrowband noise and the other for broadband noise. A series of infinite impulse response (IIR) notch filters is used to remove narrowband noise from the transmitted signal with the filter coefficients dynamically generated in accordance with a detected narrowband noise spectrum. (Other embodiments may utilize FIR or analog filters to remove the narrowband noise.) A finite impulse response (FIR) matched filter then correlates the input signal with a signal corresponding to a transmit pulse in order to remove broadband noise. (In other embodiments, matched filtering can be performed with an IIR or analog filter.) Further noise immunity is provided by dynamically adjusting the threshold at which pulses are detected from the output of the matched filter in accordance with measured noise and signal peaks.

[0014]FIG. 1 is a block diagram of the telemetry system of an external programmer. The telemetry processor 50 supervises the operation of the telemetry system, processes the data generated by it, and handles protocol functions such as timing, serial-parallel conversions, and cyclic redundancy code (CRC) checks. The telemetry processor 50 communicates with the main host processor 30 of the programmer over a host bus 40. The telemetry digital signal processor (DSP) 100 performs most of the basic processing functions for the telemetry system. It controls the transmitter, monitors the ambient noise level, and may perform some protocol functions. As described below, the DSP 100 is also responsible for matched filtering of the input samples, creating optimal notch filters for removing narrowband noise in the local noise environment, and dynamically adjusting the threshold signal level at which pulses are detected. A configuration and status channel 214 between the DSP and telemetry processor allows the telemetry processor to configure the telemetry system for a particular implantable device, monitor the received signal strength, set automatic or fixed transmitter polarities, read the wand status (i.e., presence and type), and update the DSP firmware.

[0015] The transmitter portion of the telemetry system is controlled by the DSP and includes a transmitter power supply 212, a power driver 210, and a transmit filter 208. The transmitter power supply provides voltages that are compatible with the telemetry wand antenna and provides adjustability of the transmit power by the DSP. The power driver is controlled by the DSP and generates square waves that minimize interference with surface ECG and pace detection. The transmitter filter removes high-frequency components of the power driver's waveform that may cause radiative interference with other devices. A wand antenna 205 is used for both transmitting and receiving signals. The wand style detector 206 senses both the presence of a wand and the wand type by measuring the resistance of a wand identification resistor. This allows the telemetry system to adjust the transmitter and receiver as necessary for particular types of wand antennas. The detector also causes the system to disable the transmitter if the wand is disconnected. The analog portion of the receiving circuitry includes a filter/amplifier 204 that amplifies signals received by the wand as necessary and applies the low-pass anti-aliasing filtering to the signal prior to analog-to-digital conversion by A/D converter 202. The DSP controls the filter/amplifier's overall gain to adjust for the responses of different types of wands.

[0016]FIG. 2 is a block diagram of the components making up the receiver portion of the telemetry system. The wand antenna 205 transduces a changing magnetic field intensity to a voltage which is the input signal to the analog receiver circuitry. The filter/amplifier 204 includes gain circuitry 204 a that is distributed throughout the receiver and is controllable by the DSP, and a filter 204 b that provides an anti-aliasing function with its poles distributed throughout the analog receiver circuitry. In an exemplary embodiment, a 100 KHz carrier signal is ASK modulated with a pulse train sub-carrier encoded with digital data, and the transmit pulses occur at a typical rate of 4 KHz with a pulse width between 20 and 100 microseconds, resulting in a bandwidth of the modulated carrier of approximately 10 to 150 KHz. In order to digitally demodulate the carrier waveform, the analog-to-digital converter must then sample the received signal at a rate at least equal to the Nyquist frequency of 300 KHz. In order to provide good correlation peaks in the matched filter used to detect transmit pulses and to simplify the DSP code, the analog-to-digital converter should preferably sample at a somewhat higher rate (e.g., approximately 350-400 KHz). The resolution of the A/D converter should also be at least 10 bits in order to provide dynamic range without an automatic gain control circuit. In an exemplary embodiment, a 150-kHz, seventh-order Butterworth filter provides the anti-aliasing function prior to sampling, and a 10-bit analog-to-digital converter (ADC) 202 with integrated sample and hold generates the input samples. A feedback mechanism within the analog receiver regulates a voltage bias to the receiver input which tends to remove any low frequency components from the input signal. The output of the ADC is a synchronous serial data stream which is sent to the DSP, and the DSP controls the sample rate of the ADC.

[0017]FIG. 3 is a block diagram of the functions performed by the digital signal processor 100. When the DSP receives a sample from the ADC, an interrupt is generated. The receiver interrupt handler 110 executed by the DSP processes the samples in the time domain with notch filters 112 and a matched filter 113, digitizes the presence or absence of transmit pulses via pulse detector 114, and then sends this digital data signal TEL_RX to the telemetry microprocessor 50. The receiver interrupt handler also fills a 512 element noise buffer 115 with consecutive raw input samples. When this buffer is filled, the filter generator task 120 processes the buffered data to generate new notch filter coefficients. The receiver interrupt handler then uses these coefficients to adaptively filter out narrowband noise. A noise peak detector 116 and a signal peak detector 117 detect and save peak signal values and peak noise values, respectively. These peak values are periodically processed by the threshold adjustment task 130 in order to adaptively set the threshold that the pulse detector 114 uses to digitize the serial stream.

[0018] An integer conversion routine 111 initially subtracts an offset from the input sample to convert the sample from an unsigned integer to a signed integer and remove any bias added by the analog receiver. The sample is then processed through a six biquad IIR filter 112. Each biquad is either a notch filter or a simple pass-through function so that zero to six notch filters may be active at any time. The purpose of the notch filters is to remove narrowband noise from the input signal samples. Since the presence and frequency of this noise depends on the ambient environment, the notch filter coefficients are adaptively generated in response to detected narrow band noise. The filter generator task 120 does this by processing the raw input data in the buffer 115 and periodically updating the IIR filter coefficients.

[0019] In order to obtain an optimum frequency response characteristic, the notch filters in this implementation are recursive filters (i.e., infinite impulse response) with adaptively generated filter coefficients so that the notch frequencies match the frequency peaks of a detected noise spectrum. The noise spectrum is detected by first computing a power spectrum of the input signal. The receiver interrupt handler 110 fills a 512 element buffer 115 with consecutive raw input samples. When this buffer is full, this task then scales the buffer values up to limit round-off noise in later calculations at block 121, applies a windowing function such as a Hamming window to the data to limit spectral spreading at block 122, and then discrete Fourier transforms the time domain data into frequency domain data via a Fast Fourier Transform (FFT) algorithm at block 123. The FFT output is then transformed into a power spectrum by taking the norm of the FFT output at block 124. The receiver interrupt handler then fills the buffer again, and the mean of eight consecutive power spectra is taken by block 125. This average power spectrum is then processed by noise spectrum detector 126 in order to detect narrow band noise peaks by subtracting from it a template spectrum corresponding to an expected input signal without noise. A template spectrum is pre-computed from a representative input signal generated under noise-free conditions so that when it is subtracted from the input signal spectrum, the result approximates the power spectrum of the narrowband noise alone. In order to produce a detected noise spectrum that most closely approximates the true noise spectrum, the template spectrum is scaled by a factor that reduces the total power in the detected noise spectrum to a minimal value. The frequency peaks in the detected noise spectrum are then identified and used to synthesize filters with corresponding notch frequencies to remove the noise from the input signal. The notch filters are synthesized with well-known filter synthesis algorithms by filter synthesizer 127.

[0020] An exemplary implementation of the filter coefficient updating method just described is as follows. Let P_(i) be the power spectrum of the input signal and P_(e) be the template spectrum corresponding to the noise-free signal. The detected noise spectrum P_(n) is then computed as:

P _(n) =P _(i) −R*P _(e)

[0021] where R is a scaling factor chosen to minimize P_(n). As a first approximation, R is set to a ratio of P_(i) to P_(e) computed by dividing P_(i) by P_(e) for each frequency bin, totaling up these ratios, and taking the average ratio. A successive approximation approach is then used to find the optimal scaling factor. First, values for R are found that produce a positive P_(n) and a negative P_(n), referred to as R+ and R−, respectively. Since between these two values is the value of R that minimizes P_(n), new values for R are computed as the average of R+ and R−. As each new R value is tried in the above equation, it replaces the previous value of R+ or R− according to whether P_(n) is made negative or positive, respectively. The procedure is iterated until the optimal value of R is found to result in the noise spectrum P_(n). Spectral threshold values for setting the notch filter coefficients are determined by computing the mean and standard deviation of the spectrum P_(n). In a preferred embodiment, the spectral thresholds are then set at three standard deviations above the mean. These spectral thresholds then constitute the frequency peaks used to set the notch frequencies of the notch filters 112.

[0022] Referring to block 110 of FIG. 3, the output of the notch filter stage is input to the FIR matched filter 113. The coefficients of the FIR filter 113 are designed to correlate the filtered input signal samples with samples that would be expected from a transmit pulse generated by the implantable device. This type of filter is very effective in discriminating transmit pulses from background noise and increases the range of the telemetry system. The FIR coefficients are derived by capturing a strong, noise-free transmission signal from the implantable device immediately after the samples are converted to signed integers in the receiver interrupt handler. The captured data is then manipulated so that the signal samples are reversed in their order, thus flipping them in time, and each sample is amplitude offset so the average of the samples is near zero in order to eliminate any DC component from the coefficients. The samples are then normalized so that they are fractions, with the maximum sample amplitude equal to 1.0. These fractions are then scaled so the results are in the range of −32768 to 32767 and then copied into the appropriate FIR coefficient table. With these FIR filter coefficients, the matched filter 113 performs a convolution between the input signal samples and samples corresponding to a time-reversed version of the transmit pulse expected to be generated by the implantable device, which is equivalent to performing a cross-correlation between the input signal and a transmit pulse. The output of the matched filter 113 is then compared to a threshold value (td_threshold) by the pulse detector 114. The TEL_RX signal is set high if the filtered value is above td_threshold, otherwise TEL_RX is set low.

[0023] The FIR filter output noise and signal peak values are calculated by peak detectors 116 and 117 which are then saved for processing by the threshold adjustment task 130. Signal values are discriminated from noise values based on the timing of the sampled data relative to the last transmit pulse. If a sample occurs at a time when the telemetry protocol does not allow a transmit pulse from the implantable device, then the sample is assumed to be noise, otherwise it is assumed to be a signal. These peak values are periodically processed by the threshold adjustment task 130 in order to adaptively set the value of td_threshold that the pulse detector 114 uses to digitize the serial stream.

[0024] The threshold adjustment task 130 uses the peak noise and signal values calculated by the receiver interrupt handler to update the value of td_threshold. The threshold is dynamic so that best spurious noise rejection is accomplished in noisy environments and maximum sensitivity is accomplished in noise-free environments. A local variable, min_threshold, is maintained. This variable is used to set a lower limit to the value of td_threshold. It can rapidly increase in value, but can only slowly decrease in value. If the noise peak is greater than min_threshold as determined at step 131, then min_threshold is assigned the noise peak value at step 132. This is done so that spurious noise which makes its way through the digital filters can be rapidly responded to. Note that Gaussian noise will statistically attain very large values on rare occasions, so min_threshold will track to the Gaussian noise peaks, not the average level. If the noise peak is not greater than min_threshold, then the peak value is averaged into min_threshold using a weighted moving average at step 133. The peak value is lightly weighted, so that the decay rate of min_threshold is relatively slow. This slow averaging is done because min_threshold is designed to guard against spurious noise conditions (i.e., noise absent over a short interval does not necessarily mean the noise has gone away).

[0025] The signal peak is then compared to min_threshold at step 134. If it is below min_threshold, then a transmitted signal is assumed to be absent, and td_threshold simply remains at its current value. If the signal peak is above min_threshold, then the signal peak value is averaged into a local store using a weighted moving average at step 135. The value of td_threshold is then set to half the value of this local store. Thus, the value of td_threshold tends to be one half the value of the transmit pulse peak value. The signal peak is weighted relatively heavily, so that td_threshold can react to variations in telemetry range that normally occur as the operator manipulates the wand. Note that if the calculated value of td_threshold is below min_threshold, then td_threshold is clamped to the value of min_threshold. Also, although it is desirable to rapidly change td_threshold in order to react to range variations, it is not desirable for it to change too quickly. Since the matched filtered transmit pulses have a finite slope, the threshold at which a signal is detected will affect the time at which the digitized output changes state. Since the time domain FIR filter output tends to have a shape similar to a Sinc function, rapid variations in td_threshold could detect only the main lobe for some data bits, and leading side lobes for other data bits. Certain synchronization protocols are particularly sensitive to this problem, since they use an alignment bit to establish the data window timing for the subsequent data bits. The threshold should therefore preferably be stable from when the alignment bit is detected to when the last data bit is detected. Thus, the averaging weight of the peak signal is preferably chosen to achieve the best compromise between responsiveness to range variation and threshold stability during receipt of transmitted word.

[0026] In the embodiments of the invention described above, the received signal was digitized and processed in the digital domain to derive the transmit pulses. In other embodiments, the received signal could be processed in the analog domain to remove narrowband and narrowband noise, correlate the signal with a transmit pulse by matched filtering, and detect transmit pulses with an adaptive threshold.

[0027] Although the invention has been described in conjunction with the foregoing specific embodiment, many alternatives, variations, and modifications will be apparent to those of ordinary skill in the art. Such alternatives, variations, and modifications are intended to fall within the scope of the following appended claims. 

What is claimed is:
 1. A medical device programmer comprising: an antenna to receive a signal transmitted from an implantable medical device, the signal including a carrier signal modulated with transmit pulses by amplitude shift keying, the transmit pulses generated by the implantable medical device; analog receiver circuitry coupled to the antenna; at least one filter coupled to the analog receiver circuitry, the at least one filter having frequency characteristics dynamically adjustable based on at least one peak frequency in a noise spectrum of the received signal; a noise spectrum detector, coupled to the at least one filter, to detect the at least one peak frequency in the noise spectrum; and a pulse detector, coupled to the at least one filter, to detect the transmit pulses.
 2. The programmer of claim 1, wherein the signal has a bandwidth of approximately 10 to 150 kilohertz.
 3. The programmer of claim 1, further comprising an analog-to-digital converter, coupled between the analog receiver circuitry and the at least one filter, to digitize the received signal, and wherein the at least one filter comprises at least one digital filter having dynamically adjustable filter coefficients.
 4. The programmer of claim 3, further comprising a filter synthesizer, coupled between the noise spectrum detector and the at least one filter, to synthesize the at least one filter based on the at least one peak frequency in the noise spectrum.
 5. The programmer of claim 4, wherein the at least one digital filter comprises at least one digital notch filter to remove narrowband noise from the received signal.
 6. The programmer of claim 5, wherein the at least one digital notch filter has a notch frequency corresponding to the at least one peak frequency in the noise spectrum.
 7. The programmer of claim 6, wherein the at least one digital notch filter comprises an infinite impulse response (IIR) notch filter.
 8. The programmer of claim 6, further comprising a digital matched filter, coupled between the at least one digital notch filter and the pulse detector, to remove broadband noise from the received signal.
 9. The programmer of claim 8, wherein the digital matched filter comprises a finite impulse response (FIR) filter having coefficients derived based on a transmit pulse expected to be generated by the implantable medical device.
 10. The programmer of claim 9, wherein the digital matched filter has coefficients derived based on a captured noise-free signal transmitted from the implantable medical device.
 11. The programmer of claim 9, wherein the digital matched filter is adapted to perform a convolution between the received signal and a signal corresponding to a time-reversed version of the transmit pulse expected to be generated by the implantable medical device.
 12. The programmer of claim 8, wherein the pulse detector detects the transmit pulses by comparing an output of the matched filter with a threshold value.
 13. The programmer of claim 12, further comprising: a noise peak detector, coupled to the digital matched filter, to detect peak noise values in the digitized received signal; a signal peak detector, coupled to the digital matched filter, to detect peak signal values in the digitized received signal; and threshold adjustment circuitry for adjusting the threshold value based on the peak noise values and the peak signal values.
 14. A medical device programmer comprising: means for receiving a signal transmitted from an implantable medical device, the signal including transmit pulses generated by the implantable medical device; and means for removing narrowband noise from the received signal, including: means for detecting a noise spectrum of the received signal; means for detecting frequency peaks in the noise spectrum; and means for synthesizing one or more filters based on the detected frequency peaks in the noise spectrum.
 15. The programmer of claim 14, further comprising means for removing broadband noise from the received signal by correlating the received signal with a transmit pulse expected to be generated by the implantable medical device.
 16. The programmer of claim 15, further comprising means for detecting the transmit pulses by comparing an output of the means for removing the broadband noise from the received signal with a threshold value.
 17. The programmer of claim 16, further comprising means for adjusting the threshold value based on the received signal.
 18. The programmer of claim 17, wherein the means for adjusting the threshold value comprises; means for detecting peak noise values from the received signal; means for detecting peak signal values from the received signal; and means for dynamically adjusting the threshold value based on the peak noise values and the peak signal values.
 19. A method comprising: receiving a signal transmitted from an implantable medical device, the signal including a carrier signal modulated with transmit pulses by amplitude shift keying, the transmit pulses generated by the implantable medical device; detecting a noise spectrum of the received signal; detecting one or more peak frequencies in the noise spectrum; adjusting frequency characteristics of one or more filters based on the detected one or more peak frequencies in the noise spectrum; filtering the received signal using the one or more filters; and detecting the transmit pulses from the filtered received signal.
 20. The method of claim 19, wherein adjusting the frequency characteristics of the one or more filters comprises dynamically synthesizing one or more digital notch filters to remove narrowband noise.
 21. The method of claim 20, wherein dynamically synthesizing the one or more digital notch filters comprises dynamically adjusting notch frequencies according to the detected one or more peak frequencies in the noise spectrum.
 22. The method of claim 21, wherein dynamically synthesizing the one or more digital notch filters comprises dynamically adjusting filter coefficients of one or more infinite impulse response (IIR) filters.
 23. The method of claim 21, wherein detecting the noise spectrum of the received signal comprises: computing a power spectrum, P_(i), of the received signal; and computing the noise spectrum, P_(n)=P_(i)−R*P_(e), where P_(e) is a template spectrum precomputed from a representative received signal generated under noise-free conditions, and R is an average ratio of P_(i) to P_(e).
 24. The method of claim 23, furthering comprising computing spectral threshold values for detecting the peak frequencies in the noise spectrum based on a mean and a standard deviation of P_(n).
 25. The method of claim 24, wherein computing the spectral threshold values comprises setting the spectral threshold values at three standard deviations above the mean of P_(n).
 26. The method of claim 20, further comprising filtering the received signal with a matched filter to remove broadband noise.
 27. The method of claim 26, wherein filtering the received signal with the matched filter comprises performing a convolution between the received signal and a digital signal corresponding to a time-reversed version of a transmit pulse expected to be generated by the implantable medical device.
 28. The method of claim 26, wherein filtering the received signal with the matched filter comprises deriving filter coefficients based on a transmit pulse expected to be generated by the implantable medical device.
 29. The method of claim 28, wherein deriving the filter coefficients comprises deriving filter coefficients for a finite impulse response (FIR) filter.
 30. The method of claim 28, wherein deriving the filter coefficients comprising capturing a noise-free signal transmitted from the implantable medical device.
 31. The method of claim 26, wherein detecting the transmit pulses comprises comparing an output of the matched filter with a threshold value.
 32. The method of claim 31, further comprising: detecting peak noise values in the received signal; detecting peak signal values in the received signal; and adjusting the threshold value based on the peak noise values and the peak signal values. 